This invention relates to array antenna systems and particularly to such systems wherein the required number of phase shifters or other active components is reduced by use of a coupling network interconnecting groups of antenna elements.
Prior U.S. patent application Ser. No. 594,934 entitled "Limited Scan Array Antenna Systems With Sharp Cutoff of Element Pattern," filed July 10, 1975 and now U.S. Pat. No. 4,041,501 which is assigned to the same assignee as the present invention, discloses an array antenna system wherein a coupling network interconnects groups of array antenna elements. Wave energy signals supplied at the input of any element group are coupled directly to the elements of that group and are also supplied through the coupling network to selected elements in the remaining element groups of the array. As a result, the array aperture is provided with an excitation, which closely approximates an ideal excitation to produce an effective element pattern wherein substantial radiation occurs only in a desired region of space. The specification and drawings of the prior application are incorporated into this application by reference.
FIG. 16 of the prior application discloses a technique for shifting the angular location of the effective element pattern of the array by providing linear increments of phase adjustment between the antenna elements and the coupling networks. As illustrated in FIG. 15 of that prior application, the effective element pattern can be displaced, for example, to one side of the broadside axis of the array. This prior technique for shifting the effective element pattern also angularly shifts the radiated array pattern by the same amount, since the phase adjustments are provided immediately adjacent to the radiating elements. As a result, if the pahse adjustments illustrated in FIG. 16 of the prior application are utilized in an array antenna such as shown in FIG. 6 of that application, both the antenna element pattern and main beam of the antenna are shifted in space. If phase shifters 13 of the antenna are set to radiate a beam in the broadside direction, the phase adjusting line lengths 74 will cause a shift in the direction of the antenna beam off the broadside axis by the same angular displacement as is given element pattern 77.
A similar effect results when the phase adjustment line lengths 75 are provided in an antenna having an input commutation switch, such as is shown in FIG. 7 of the prior application. In this case, the antenna radiates a pattern wherein the radiated frequency varies as a function of angle from the broadside axis of the array. The phase adjustments 75 will shift not only the effective element pattern, but also the frequency coding of the radiated signal.
FIG. 2 illustrates a microwave landing system environment wherein the present invention is particularly useful. A navigation antenna 52 of the type described in the referenced prior application is located adjacent an airport runway 54. Near the approach of runway 54, there is located uneven terrain 56. When an aircraft 58 is approaching runway 54, it may receive a signal 66 directly from antenna 52, and may also receive a signal 64 which has been reflected off the uneven terrain 56. In such an installation, it is particularly desirable to shift the location of the effective element pattern 60 of antenna 52 such that the radiation in the angular direction of the uneven terrain 56 is reduced, thereby to reduce navigation error resulting from multipath signal 64. In the event angular shifting of element pattern 60 is achieved by the method shown in FIG. 16 of the prior application, there will also be a shift in the direction of the antenna beam 62. If antenna 52 is used in a "scanning beam" landing system wherein a narrow antenna beam is moved through space at regular time intervals, the shift of antenna beam 62 will be manifested by an angular change in the direction of the antenna beam at any particular instant of time. In the event antenna 52 is used in a "Doppler" landing system, making use of a commutator arrangement such as shown in FIG. 7 of the prior application, antenna beam 62 represents the signal which is detected by a narrow bandwidth receiver, since antenna 52 radiates into the entire angular region defined by element pattern 60 with a radiation pattern wherein radiated frequency varies with angular direction. In a Doppler system, the prior art pattern shifting technique will result in a change in the angular frequency coding, thereby causing a frequency change in the radiated signal at any particular angle.
Since the prior art technique of changing the angular position of the effective element pattern results in a change in the frequency or time coding of the radiated signal, such modification to the antenna system to accommodate uneven terrain at a particular installation location results in additional complexity in the navigation equipment. Either the receiver in aircraft 58 must be advised of, and perform a correction calculation for, the resulting change in navigation coding or the coding mechanism of antenna 52 must be adjusted to correct for the change in the frequency or time coding of the radiated signal.
Another problem with the prior art technique of providing a phase shift adjustment at the inputs of the particular antenna elements is that such a phase adjustment eliminates the possibility of having uniform antenna element groups, each group consisting of elements, power divider, interconnecting transmission lines, couplers, and interconnecting networks, which could be produced as a modular unit. The element pattern steering technique of the prior application required different phase adjustment for each element. This eliminated the possibility of uniform modular construction. Further, the amount of phase adjustment could be very large for a large array.
It is therefore an object of the present invention to provide an array antenna system having an element pattern confined to a selected region of space wherein the angular location of the element pattern can be adjusted.
It is a further object of the present invention to provide such an antenna system wherein the adjustment of the angular location of the element pattern results only in an amplitude change of the array antenna pattern.
It is a still further object of the invention to provide such an antenna system wherein modular construction may be implemented to provide substantially identical element and network groups.
It is a still further object of the invention to provide phase adjustable microstrip transmission line useable in such antenna systems.